Resistive-inductive wall amplifier tube



BY ya 2'@ A. V. HAEFF ET AL.

RESISTIVE-INDUCTIVE WALL AMPLIFIER TUBE Filed Oct. 1, 1952 May 21, 1957United States Patent O 2,793,315 REsrsTrvE-INDUCTIVE WALL AMPLIFIER TUBEAndrew V. Haeff, Pacific Palisades, and Charles K. Birdsall, LosAngeles, Calif., assignors, by mesne assignments, to Hughes AircraftCompany, a corporation of Delaware Application October 1, 1952, SerialNo. 312,568 13 Claims. (Cl. S15-3.6)

This invention relates to microwave amplifier tubes, and moreparticularly, to an improved electron stream amplifier tube.

The present invention is directed to an electron stream amplifier tubeof the type which is capable of amplifying microwave energy by virtue ofthe interaction of the electrons of a modulated electron stream withelectromagnetic fields produced by currents induced by the modulatedelectron stream in an impedance wall disposed contiguous to the stream.

The present electron stream amplifier tube may be considered animprovement over that disclosed in a copending application for patententitled, Electron Stream Amplifier Tube, by Andrew V. Haeff, filedApril 12, 1952, Serial No. 282,000, now Patent No. 2,740,917, datedApril 3, 1956. In the copending Haeff application, an electron streamamplifier tube is described having three sections in addition to theusual electron gun and collector electrodes. The first section is arelatively short input structure whose function is to transform signalenergy into modulations of the electron stream. The second section,referred to as an impedance member, is a structure surrounding thestream and having walls of resistive or inductive material. By way ofexample, the impedance member may consist of a long piece of glasstubing having a resistive coating on its inner surface, the electronstream being projected through the hollow portion of this structure. Theoriginal modulations of the stream are then amplified throughinteraction between the modulated electron stream and the electricfields produced by the currents induced in the resistive or inductivewall material. The third section of the device is an output structurewhere the amplified signal energy of the electron stream is convertedinto a useful output signal.

As emphasized in the Hae application, the resistive or inductive wall,even in its simplest form, does not present a pure resistive impedanceto the electron stream, but inherently includes distributed capacitancethat is undesirable since it acts as a low impedance path for themodulations of the stream, thus decreasing the available gain.

The present invention discloses a novel electron stream amplifier tubefor amplifying microwave signals having substantially no distributedcapacitance effects in the resistive wall. Alternatively and, ifdesired, the tube may be designed to have a resistive-inductive wall, oran inductive wall. This is accomplished by positioning a highlyconducting surface, such as a metallic wall, at a distance between aneven and an odd multiple of an electrical one-quarter wavelength at thesignal frequency behind the resistive surface exposed to the electronstream; that is, the spacing between the conductive and resistivesurfaces should be less than one-quarter, or between twoquarters andthree-quarters of a wavelength, and so on. In order to decrease thedistributed capacitance of the wall structure, or to obtain an inductiveimpedance effeet, it is necessary that a wave be able to proceed from2,793,315 Patented May 21, 1957 ICC the resistive surface to themetallic wall or conductive surface and be reflected back to theresistive surface at a velocity slightly less than that of the electronstream without being unduly attenuated.

This is made possible by inserting a slow-wave material between theresistive wall and the reflecting or conductive wall which allows a waveexcited by the electron stream to be propagated without appreciableattenuation transversely to the direction of the electron flow. Asuitable slow-wave material for use in the tube of the present inventionis defined as one wherein electromagnetic waves are propagated throughan unbounded sample of the material at a phase velocity equal to or lessthan the electron stream velocity. In the event such a material is notused, there will be reactive attenuation transverse to the electronflow, similar to propagation in a waveguide below its cut-off frequency.Further, a suitable slow-wave material may be obtained artificially bythe choice of a material having suitable dielectric and magneticcharacteristics. The effect of this transverse electromagnetic wavetraversing such a material, as previously mentioned, is to decrease theeffect of the distributed capacitance of the resistive wall or to placean inductive impedance electrically in parallel with the resistive wall;this transverse electromagnetic wave is not to be confused with an axialslow-wave having an axial velocity synchronized with the velocity of theelectrons of the stream for the purpose of obtaining amplification.

It is, therefore, an object of this invention to increase the gain of anelectron stream amplifier tube by decreasing the distributed capacitanceinherent in the resistive wall of the impedance member of the tube.

Another object of this invention is to provide a member for presenting apurely resistive impedance to the electron stream of an electron streamamplifier tube.

An additional object of this invention is to provide a member forpresenting a resistive-inductive impedance to the electron stream of anelectron stream amplifier tube.

A further object of this invention is to provide a member for presentinga substantially pure inductive impedance to the electron stream of anelectron stream amplifier tube.

A still further object of this invention is to provide an electronstream amplifier tube capable of amplifying microwave signal energy ofbrood bandwidth wherein the admittance presented to the electron streamby the impedance member of the tube is capable of producing optimumamplification with tubes of this type.

The novel features which are believed to be characteristie of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawing in which an embodiment of the invention isillustrated, by way of example. It is to be expressly understood,however, that the drawing is for the purpose of illustration anddescription only, and is not intended as a definition of the limits ofthe invention.

Fig. 1 is a vertical cross sectional view of the tube with associatedcircuits; and

Figs. 2 and 3 are equivalent circuit diagrams.

Referring to Fig. 1, the amplifier tube comprises an evacuatedcylindrical glass envelope 2 with an enlarged portion at the leftextremity as viewed in the drawing, which houses an electron gun 3 forproducing an electron stream. The electron gun 3 has a cathode 4 with aheater 6, a focusing electrode 8, and an anode 10. Heater 6 is connectedacross a source of potential, such as a battery 16, the negativeterminal of which may be connected to cathode 4, as shown. Cathode 4 andfocusing electrode 8 are connected together and are, in turn,

connected to ground. Anode is connected to the movable arm of apotentiometer 24, which is connected across a source of potential 26.the negative terminal of which is connected to ground. Potentiometer 24is used to adjust the potential applied to anode 10 which functions as acontrol element for determining the current of the electron stream. Apotential of 500 volts with respect to ground is representative of thepotential normally applied to anode 10.

Positioned axially about the electron stream in the direction of theelectron ow, are a matching ferrule 12 connected by a lead 14 to aconducting input helix 30, an impedance member 38, a conducting outputhelix 44 connected by a lead to a matching ferrule 46 and a collector47.

Matching ferrule 12 and input helix 30 are maintained at an appropriatepositive potential with respect to ground by connecting it over aconductor 39 and a potentiometer 24 to the positive terminal of abattery 26. A potential of 1000 volts with respect to ground isrepresentative of the voltage normally impressed on ferrule 12 and inputhelix 30.

Under normal operation, a growing electromagnetic wave employed tomodulate the electron stream is propagated on the helix 30 along withthe flow of stream electrons. A coating of resistive material 31, suchas Aquadag. is applied on the outside of envelope 2 about the last fewturns of the helix 30 to terminate this wave. Termination of the wave iseffected by the resistive coat ing 31 being inductively coupled to thewave in such a manner that currents induced by the wave flow within theresistive material 31 thereby dissipating the energy of the wave. lnaddition, helix 30. being axially aligned with the electron streamemanating from electron gun 3, generally has an inner diameter that issubstantially equal to the inner diameter of the ferrule 12 so that thestream electrons pass as close to the helix as possible without beingintercepted by the latter. A material, such as tungsten, is suitable formaking the helix, the main prerequisite being that the helix retains itsform, especially with respect to its pitch and diameter.

An input waveguide 34 is mounted so that lead 14, connecting input helix30 to electrode 12, is located approximately one-quarter wavelength fromtermination 36 which comprises a shorting surface adjustable in positionby a matching element 35. Lead 14 is also disposed so as to be parallelto the electric field in waveguide 34 to allow maximum transfer ofenergy from the waveguide to lead 14. The matching element 35 provides ameans for adjusting the distance from lead 14 to termination 36 so thatthe voltage induced in lead 14 is at an optimum value.

Cylindrical collar 18 is concentric with ferrule 12 and extends for adistance of roughly one-quarter Wavelength. Since collar 18 isopen-circuited at the farthest extremity with respect to its associatedwaveguide, an apparent shorting plane is produced at the inner surfaceof waveguide 34.

A preferred embodiment of impedance member 38 is tubular in form.Impedance member 38 is axially aligned with helix 30, the insidediameters of the two being approximately equal. Impedance member 38comprises a tubular element 60, a resistive coating 61 deposited on theinner surface of element 6l), and a highly conductive coating 62deposited on the outer surface of element 60.

Tubular element 60 may be fabricated of several types of materialshaving suitable electromagnetic wave propagating characteristics. lngeneral, if VKv represents the voltage (in kilovolts) by which thestream electrons have been accelerated, then the necessary prerequisitefor the slow-wave material of tubular element 60 is that the product RERbe greater than or equal to wherein y, is the permeability relative tothat of free space, and fR is the relative dielectric constant, asmeasured for propagation transverse to the electron stream direction. Inaddition to the foregoing, it is desirable that the material used haverelatively low conductive losses, particularly in the case of materialshaving a high dielectric constant. Materials satisfying the aboverequirements may be divided into three groups, namely, materials havinga high dielectric and a low permeability constant; materials having ahigh permeability constant; and materials having artificially produceddielectric and magnetic characteristics, for example, nonhomogeneousmaterials.

Included in the class of materials having a high dielectric constant anda low permeability constant are the titanate bodies which includetitanium dioxide, calcium titanate, strontium titanate, and bariumtitanate, the relative dielectric constant of these materials being,respectively, of the order of 100, 150, 250 and 1000. Titanate bodiesmay be processed so as to have physical characteristics of a ceramicmaterial which may be readily produced in the tubular shape as requiredfor element 60.

Secondly, materials having a high permeability constant at microwavefrequencies include ferrites, the general chemical formula for ferritesbeing R'FezOs, wherein R represents a metal such as magnesium or copper.The permeability of ferrite materials exhibits a magnetic resonance atfrequencies which are in the microwave range, thus providing suticientlyhigh values of permeability to make their use practical as a slow-wavematerial in the present invention. It is preferred to use a materialhaving a high permeability constant to decrease the velocity of anelectromagnetic wave in that higher losses can be tolerated than wouldbe in the case with high dielectric constant materials without adverselyaffecting the impedance.

The third type of slow-wave material is produced artiiically bydistributing small pieces of resonant metallic particles throughout adielectric or magnetic material. As is commonly known, metallicparticles are said to be resonant at a particular frequency when onedimension `approximates one-half wavelength at the particular frequency.At frequencies just less than the particular frequency at whichresonance occurs, the metallic particles exhibit inductive reactance.Materials of this class together with the techniques for making them arewell known in the art and need no further description. See, for example,an article entitied, Metallic delay lenses" by W. E. Koch which appearson pages 58-82 of vol. 27, No. l of the Bell Systems Technical Journalfor January 1948, published in New York, New York.

Impedance member 38 may bc manufactured. for example, by using amaterial such as strontium titanate in u ceramic form shaped as tubularelements, the thickness of the walls being made equal to the length ofan electrical one-quarter wavelength at the signal frequency. When thethickness of element 60 is equal to one-quarter wavelength, it presentsa purely resistive impedance to the electron stream. As statedpreviously, the wall thickness may also be made any odd multiple ofone-quarter wavelength. Alternatively, in case a resistive-inductive orpure inductive impedance is desired, the wall thickness may be made lessthan an odd multiple of one-quarter wavelength and greater than an evenmultiple of oncquarter wavelength. Highly conductive coating 62 may thenbe provided by plating or evaporating a metal, such as silver, on theouter surface of element 60.

The resistive coating 61 is then deposited on the inner surface oftubular element 60. Resistive coating 6l may consist of a layer ofstannous oxide formed by reacting stannous chloride with a suitableagent on the surface material. Resistive coatings of this type are muchthinner than the skin depth which is representative of the penetrationof microwave energy and, hence, exhibits a arsenic` radio-frequencyresistance that approximates that for direct currents. For obtainingmaximum gain, the surface resistivity must be chosen to have a valuedepending upon the desired operating frequency, the velocity of theelectron stream and the dielectric constant of the material. By surfaceresistivity is means the actual resistance per unit surface area to anelectrical wave. For stannous oxide on a dielectric surface, the directcurrent resistance per unit area approximates the value of surfaceresistance presented to the wave, since its thickness is much less thanthe usual skin depth encountered at the frequencies used.

The choice of surface resistivity of the wall required for obtainingmaximum amplification for unit length of the tube depends upon theoperating frequency, the separation between the electron stream andresistance wall, the electron velocity, and the dielectric constant ofthe Wall. Furthermore, the surface resistivity may be very high when aresistive-inductive wall is desired, the value of resistivity in thiscase being many times that used with a purely resistive wall. For apurely inductive wall, the resistivity must be as high as is compatiblewith collecting stray electrons and maintaining a uniform staticpotential over the entire length of the wall. Obviously, this structureprovides a purely inductive wall only when infinite resistivity isapproached, which limit can never be attained due to the above staticrequirements. In normal practice, values of surface resistivity will befound to range from 500 to 10,000 ohms per square centimeter, or even ashigh as, for example, 100,000 ohms per square centimeter for theinductive wall. The resistive coating 61 is normally maintained at apotential approximately equal to that of the matching ferrule 12. Forconvenience, the same battery 26 is used to apply a potential of theorder of 1000 volts through leads 40 and 42 to both extremities ofresistive coating 61.

The output portion of the amplifier tube comprises the output helix 44connected by lead 20 to matching ferrule 46 and collector electrode 47,all maintained at a potential equal to the potential of matching ferrule12. This is accomplished by means of wires 48 and 50 and source 26, thelatter being of the order of 1000 volts.

The construction of helix 44 is identical to that of helix 30, and it isterminated in a manner similar to helix 30 by applying a coating ofresistive material 43, such as Aquadag, to the outside of envelope 2.

The construction of the output waveguide 52 is identical to that of theinput waveguide 34. It has a matching element 53 for positioning atermination 54, similar to matching element 35 in the input waveguide34. A cylindrical collar 45 extends concentric with ferrule 46 for adistance of approximately one-quarter wavelength producing an apparentshorting plane on the inner surface of output waveguide 52. Although arather specitic means has been disclosed for coupling signal energy toand from the disclosed impedance wall amplifier tube, other expedients,such as replacing the input and output waveguides with coaxial lines maybe used. Also, the input and output helices may be replaced by resonantcavities, or .a grid may be used to directly modulate the stream ofelectrons at the input, and a screen and anode similar to those of atetrode can be used to extract the signal energy from the electronstream at the output end. In addition, the disclosed invention need notbe restricted to the particular geometrical configuration described asthe electrons can be made, for example, to ilow along any desired curvedpath merely by the utilization of appropriate electrostatic and magneticfields.

A solenoid 56 is axially positioned symmetrically about the completelength of the glass envelope 2. An appropriate direct current ismaintained by a battery 57 in solenoid 56 to produce a magnetic fieldwhich may be of the order of 500 gauss. The tield extends through theentire length of the tube and is parallel to its longitudinal axis. Thepurpose of this magnetic eld is to keep the electron stream focused orconstrained throughout the active length of the tube.

In its operation, an input microwave signal is applied through inputwaveguide 34, inducing a signal potential on lead 14. The matchingelement 35 is adjusted to produce maximum signal voltage at the inputhelix 30. As in conventional traveling wave tubes, the axial phasevelocity of the traveling wave through the helix is a fraction of thevelocity of light, the actual phase velocity being determined by thepitch and diameter of the helix. The velocity of the electron stream isusually adjusted so that it is slightly greater than the phase velocityof the wave passing through the helix. The interaction of the electronstream and the wave on the helix results in a density and velocitymodulation of the electron stream. Resistive coating 61 of the impedancemember 38 has a resistance such that an electric wave propagated throughmember 38 is very highly attenuated in the absence of the electronstream because of the relative dimensions of member 38 with respect to awavelength at the frequency of the microwave signal. Hence, nearly allof the energy transmitted along the axis of member 38 is in the form ofa space charge wave propagated by the electron stream, the signal energyexisting in the form of electron bunching of the stream electrons.Currents induced by the hunched electron stream in the resistive coating61 and in the slow-wave material 60 produce electric fields which act onthe stream electrons so as to increase still further the electronbunching, an increase in the electron bunching being equivalent to anincrease in the signal amplitude. Thus, the magnitude of the modulationof the electron stream representative of the signal amplitudecontinuously increases as the stream electrons move along contiguouslyto the resistive coating 61.

To explain more adequately the functioning of the present invention, ananalogy may be made to a shorted transmission line. As is commonlyknown, the input impedance of the transmission line is a function of theelectrical distance to the shorted termination and the losses of theline. In the case of a quarter wavelength shorted transmission line, theinput impedance is essentially infinite, provided there are no losses.Similarly, in the present invention, a wave is propagated exterior tothe resistive coating 61 to a reiiecting surface provided by the highlyconductive coating 62, thus producing any desired reactive impedance inthe plane of resistive coating 61 dependent upon the spacing and theintervening attenuation of the reilected wave. As in the case oftransmission lines, the impedance in the plane of resistive coating 61can be made inductive by decreasing the spacing between resistivecoating 6l and conductive coating 62 below that corresponding to anelectrical one-quarter wavelength, and made capacitive by increasing thespacing beyond an electrical one-quarter wavelength. Hence, in order toobtain an inductive impedance, it is only necessary to use anappropriate spacing which is just less than an odd multiple of anelectrical one-quarter wavelength. The resistive coating 6l, of course,is electrically in parallel with this inductance because the coating isphysically coincident with the surface at which this inductive impedanceappears. These paralleled impedances determine the actual impedancewhich impedance member 38 presents to the electron stream.

The aforementioned increasing space charge wave is propagated axiallyalong member 38 by the electron stream and emerges at the far end whereit induces a voltage on helix 44, transforming a portion of the signalenergy in the electron stream into the form of an electromagneticgrowing wave on the output helix. The stream electrons, as they proceedtoward ferrule 46, continue to impart energy to the growing wave on thehelix. After transferring most of the signal energy from the stream tothe helix, the electrons are collected by collector electrode 47. Theelectric field induced by the electromagnetic wave on lead 20 connectedto the output helix 44 is parallel with the electric field of thefundamental mode desired to be excited in the waveguide, hence,amplified signal energy is transferred from the output helix 44 to theoutput waveguide 52.

An analysis of impedance wall amplifiers indicates that amplification ofa wave in decibels per unit length is proportional to the imaginary partof the quantity where Y: Yw )'Bi is an admittance made up of theparallel connection of the wall admittance, Yw, and the capacitivesusceptance, Bs, of the space occupied by the electron stream, In is thecurrent in the electron stream, Vo is the potential through which thestream electrons are accelerated, e is the absolute dielectric constant,and w is the angular frequency of the amplified wave.

Reference is made, for example, to Waves in Electron Streams andCircuits, by J. R. Pierce, Bell System Technical Journal, July 1951.Examination of the foregoing expression representative of amplificationshows that when Y is purely capacitive, there will be n0 amplification.On the other hand, for Y small and purely inductive, there will be alarge gain, and for Y resistive-capacitive, resistive, orresistive-inductive, there will be gain, an increased amount of gainbeing associated with the latter admittance.

Inasrnuch as the capative susceptance, Bs, of the space occupied by theelectron stream never vanishes, a small amount of inductive susceptanceadded by an inductive wall may still leave admittance Y capacitive.Also, a large inductive susceptance may cancel susceptance BS entirely,leaving admittance Y purely resistive provided that the Wall admittance,YW. has a resistive part. A still larger inductive susceptance wouldmake admittance Y resistive-inductive.

In accordance with the present invention, the admittance, Y, obtained bycombining an inductive reactance, in parallel with the resistive coating61 having a very large resistance per unit area, results in what isessentially a large inductance. As previously mentioned, an inductivercactance in the plane of resistive coating 61 may be produced bypositioning the electromagnetic refiecting surface 62 just less than anodd number of electrical quarter wavelengths from resistive coating 61with slow-wave material 60 disposed in the intervening space. The gainper unit length of a tube incorporating such an impedance memberpresenting a high inductive reactance to the electron stream may then beof the order of tens of decibels per centimeter for currents andvoltages well Within the ranges encountered in practice.

To explain more adequately the function of the reflecting surfaceprovided by coating 62 of impedance member 38, reference is made toFigs. 2 and 3 wherein equivalent circuit diagrams arel illustrated forthe impedance presented to the electron stream by an impedance memberwith and without the reflecting surface, respectively.

Referring to Fig. 2, there is shown an equivalent circuit diagram of theimpedance presented to the electron stream by resistive coating 61acting alone, resistive elements 86, 87, 88 and 89 representing, inlumped parameter form, the series resistance of resistive coating 61,and tubular elements 80, 81, 82, 83 and 84 representing the electricalcoupling of the electron stream to the walls. In addition to theforegoing, there is also a distributed series capacitance betweenadjacent tubular elements of resistive coating 61 which is representedin lumped parameter form by capacitors 91, 92, 93 and 94. It is to benoted that this capacitance would exist even in the absence of theresistive coating 61, the capacitance being that of the space along theelectron stream.

In order to obtain maximum gain in an electron stream amplifier tube, itis necessary to maintain the impedance presented to the electron streamas high as possible consistent with the frequency bandwidth desired. Itcan readily be seen by inspection of Fig. 2 that the admittancepresented to the electron stream consists of conductance plus capacitivesusceptance. To increase the gain of the tube, it is necessary todecrease the capacitive susceptance in Jaralle! with the resistance. Onemeans of accomplishing this, as explained previous-ly, is to position ametal Wall one electrical one-quarter wavelength or less behindresistive coating 61, in addition to interposing a slow-wave materialbetween the resistive coating and the highly conductive metal wall whichacts as a reflector. In reality, this wave reflection has the samefunction as a parallel resonant circuit in that it reduces thecapacitance mentioned in one frequency range, cancels the capacitancecompletely at one predetermined frequency, and places an inductiveimpedance in parallel with the resistance of the coating in anotherfrequency range. The structure, therefore, may be illustrated as aseries of parallel elements as shown in Fig. 3, that is, inductiveelements 96, 97, 98 and 99 in parallel, respectively, with capacitors91, 92, 93 and 94, and resistive elements 86, 87, 83 and 89.

Since it has been previously pointed out in the Haeff application thatit is preferable to present a resistiveinductive impedance to anelectron stream rather than a resistive-capacitive impedance, anelectron stream amplified tube embodying the disclosed invention couldbe designed to be resonant near the upper limit of the frequency `rangeto be amplified by the tube in that amplification is greater in theresistive-inductive regions. In this manner, a resistive-inductiveimpedance is presented to the electron stream throughout the rangs offrequencies required for a given signal, which results in a uniformamplification of all components of the signal being amplified.

What is claimed as new is:

l. In an electron stream tube for amplifying a space charge wavepropagated by an electron stream, an element for presenting an impedanceto the electron stream comprising a wall composed of a slow-wavematerial having a substantially uniform thickness of substantially anodd multiple of one-quarter of a wavelength in said material at thefrequency of said wave, said wall having a first surface positionedcontiguous to the electron stream and an opposite surface; a resistivecoating deposited on said first surface whereby said space charge wavepropagated by said electron stream induces currents in said resistivecoating to generate concomitant electric fields which interact with saidelectron stream to amplify said space charge wave; and a conductivecoating deposited on said opposite surface for presenting, inconjunction with said wall composed of a slow-wave material, a highirnpedance in parallel with said resistive coating to said electronstream.

2. The element for presenting an impedance to the election stream asdefined in claim` l wherein the product of the relative dielectric andpermeability constants of said slow-wave material with respect to freespace is not less than 256 divided by the kilo electron volts throughwhich the electrons comprising said electron stream are accelerated.

3. In an electron stream tube for amplifying a space charge wavepropagated by an electron stream, an element for presenting an impedanceto the electron stream comprising a wall composed of a slow-wavematerial having a predetermined dielectric and permeability constant anda substantially uniform predetermined thickness, said wall having a rstsurface positioned contiguous to the electron stream and an oppositesurface; a resistive coating deposited on said first surface wherebysaid space charge wave propagated by said electron stream inducescurrents in said resistive coating to generate concomitant electric eldswhich interact with said electron stream to amplify said space chargewave; and a conductive coating deposited on said opposite surface forpresenting to said electron stream in conjunction with said slow-wavematerial, in parallel with said resistive coating an impedancedetermined by said predetermined thickness of said wall.

4. The element for presenting an impedance to the electron stream asdefined in claim 3 wherein said predetermined thickness of said wall isgreater than an even multiple of one-quarter of an electrical wavelengthof said wave and less than an odd multiple of one-quarter electricalwavelengths.

5 An electron stream tube for amplifying microwave signals, said tubecomprising means for producing an electron stream; means for directingsaid electron stream along a predetermined path; means for modulatingsaid electron stream in response to microwave signals to produce acorresponding space charge wave propagated along said path by saidelectron stream; a member having for presenting an impedance to saidelectron stream, said member a resistive surface extending contiguous tosaid modulated electron stream for at least several wavelengths of saidspace charge wave whereby said space charge wave induces currents insaid resistive surface to generate concomitant electric fields whichinteract with said electron stream to amplify said space charge wave, anelement having a conducting surface positioned a predetermined distancefrom and parallel to said resistive surface, and a slow-wave materialinterposed between said resistive surface and said conducting surface,whereby said conducting surface in conjunction with said slowwavematerial presents a predetermined impedance dependent upon theelectrical distance between said resistive surface and said conductingsurface to said electron stream to increase the amplification of saidspace charge wave; and output means coupled to said electron stream forderiving a microwave output signal from said amplified space chargewave.

6. The electron steam tube as defined in claim 5 wherein saidpredetermined distance between said conducting surface and saidresistive surface is greater than an even multiple of one-quarterelectrical wavelengths and no more than one-quarter odd multiple ofelectrical wavelengths.

7. An electron stream tube for amplifying microwave signals, said tubecomprising means for producing an electron stream; means for modulatingsaid electron stream in response to microwave signals to produce acorresponding space charge wave propagated by said electron stream; amember for presenting an impedance to said electron stream having aresistive surface contiguous to said modulated electron stream for atleast several wavelengths of said space charge wave, said space chargewave inducing currents in said resistive surface to generate concomitantelectric fields whereby said electric fields interact with said electronstream to produce amplification of said space charge wave, a conductingsurface disposed substantially an odd multiple of one-quarter electricalwavelengths at the frequency of said microwave signal from saidresistive surface, and a slow-wave material interposed between saidresistive surface and said conducting surface, whereby said conductingsurface, in conjunction with said slow-wave material, presents a highimpedance in parallel with said resistive surface to said electronstream; and output means coupled to said electron stream for deriving amicrowave ouput signal from said amplified space charge wave.

8. In an electron stream tube for amplifying a spacecharge wavepropagated by an electron stream, an element for presenting an impedanceto said electron stream comprising a wall composed of a slow-wavematerial having a thickness greater than an even multiple of electricalone-quarter wavelengths in said material at the frequency of said waveand no more than an odd multiple of electrical one-quarter wavelengths,said wall having a first surface positioned contiguous to said electronstream and an opposite surface; a resistive coating deposited on saidfirst surface for maintaining said surface at a uniform potential; and ahighly conductive coating deposited on said opposite surface forpresenting an inductive impedance to said electron stream.

9. In an electron stream tube for amplifying a space charge wavepropagated by an electron stream, an element for presenting an impedanceto said electron stream comprising a wall including a material ofpredetermined dielectric and magnetic characteristics and having asubstantially uniform thickness of an odd multiple of onequarterwavelengths in said material at the frequency of said wave, said wallhaving a first surface positioned contiguous to the electron stream andan opposite surface; a resistive layer disposed on said first surfacewhereby said space charge wave propagated by said electron streaminduces currents in said resistive layer to generate concomitantelectric fields which interact with said electron stream to amplify saidspace charge wave; and a conductive layer disposed on said oppositesurface for presenting a high impedance in parallel with said resistivelayer to said electron stream.

l0. The electron stream tube for amplifying microwave signals as definedin claim 7 wherein said resistive surface of said member is disposedconcentrically about said modulated electron stream.

ll. An electron stream amplifier tube for amplifying microwave signals,said tube comprising means for producing an electron stream; means fordirecting said electron stream along a predetermined path; means formodulating said electron stream in response to microwave input signals;an element for amplifying the modulations of said electron stream, saidelement including a hollow dielectric cylinder of uniform thicknessdisposed concentrically about and contiguous to said path, a resistivecoating disposed on the inner surface of said cylinder, and a conductivecoating disposed on the outer surface of said cylinder; and output meanscoupled to said electron stream for deriving a microwave output signalfrom the amplified modulations of said electron stream.

l2. An electron stream amplifier tube for amplifying microwave signals,said tube comprising means for producing an electron stream; means fordirecting said electron stream along a predetermined path; means formodulating said electron stream in response to microwave input signals;a member having a resistive layer disposed on `said memberconcentrically about and contiguous to said path, said modulationsinducing currents in sai-d resistive layer which generate electricfields, said fields interacting with said electron stream to amplifysaid modulations, a dielectric wall of uniform thickness disposedexterior to and in contact with said resistive layer, and a conductivelayer disposed on the outer surface of said dielectric wall, theportions of said electric fields, exterior to said resistive layer,propagating radially outwards to said conductive layer whence saidfields are reflected back to said resistive layer to cancel the spatialcapacitance exterior thereto thereby increasing the amplification ofsaid modulations; and output means coupled to said electron stream forderiving a microwave output signal from the amplified modulations ofsaid electron stream,

13. An electron stream amplifier tube for amplifying microwave signals,said tube comprising means for producing an electron stream; means fordirecting said electron stream along a predetermined path; means formodulating said electron stream in response to microwave input signals;a member for presenting a resistiveinductive impedance to said electronstream to amplify said modulations, said member having a resistive layerdisposed on said member concentrically about and contiguous to saidpath, said modulations inducing currents in said resistive layer whichgenerate electric fields, said fields interacting with said electronstream, a dielectric wall of uniform thickness disposed exterior to andin contact with said resistive layer, and a conductive layer disposed onthe outer surface lof said dielectric wall, the portions of saidelectric fields, exterior to said resistive layer, propagating radiallyoutwards to said conductive layer whence said fields are reected back tosaid resistive layer to produce an inductive impedance in paralleltherewith thereby increasing the amplification of said modulations; andoutput means coupled to said electron stream for deriving a microwave`output Signal from the amplified modulations of said electron stream.

King Apr. 16, 1940 Cassen Dec. 8, 1942 12 2,367,295 Llewellyn Jan. 16,1945 2,584,802 Hansell Feb. 5, 1952 2,602,148 Pierce July 1, 19522,611,101 Wallauschek Sept. 16, 1952 2,616,990 Knol et al. Nov. 4, 19522,630,547 Dodds Mar. 3, 1953 2,636,148 Gorham Apr. 2l, 1953 2,652,513Hollenberg Sept. l5, 1953 2,661,441 Mueller Dec. l, 1953 FOREIGN PATENTS969,267 France May 17, 1950 OTHER REFERENCES Article by Von Hippel etal., pp. 1097-1109, Industrial and Engineering Chemistry, for November1946, v01. 38, No. 1l.

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